Single package epoxy resin systems conventionally include a latent curing agent, typically dicyandiamide. This curing agent requires a long cure period, even at high temperatures. For example, it can be demonstrated from differential scanning calorimetric studies that the dicyandiamide-epoxy system (e.g., Epon 828, Shell Chemical Company), without the presence of an accelerator, exhibits an onset cure temperature at 193.degree. C. Normally such mixture has to be cured above 180.degree. C. for at least 30 minutes to obtain a cured thermoset for practical applications (adhesives, coatings, and sealants). To increase the curing speed and to reduce the curing temperature, curing accelerators such as imidazoles and ureas have been incorporated into epoxy-dicyandiamide systems. Due to the basic nature of imidazoles, the prepared one-package thermosetting materials normally run into storage stability problems.
(Note: "Onset cure temperature" is a term peculiar to the thermoset resin art. Consider the curing curve. The curve is substantially flat (base line) until the resin reaches a temperature at which curing begins. Then the curve slopes sharply upward, as a measure of the amount of curing that has taken place. "Onset cure temperature" is the intersection of tangents drawn respectively to the base line and to the sharp upward curve. A consideration of the geometry of the curing curve and this intersection will show that curing occurs before onset cure temperature is reached.)
Various urea derivatives have been reported as epoxy accelerators, with and without dicyandiamide, e.g. (all references are to Chemical Abstracts):
N-phenyl-N',N'-dimethyl urea; 78(10):59299t. PA1 Phenylurea or 1,3-diphenylurea; 83(22):180415j. PA1 Substituted urea or thiourea; 85(12):78946n. PA1 Substituted urea (Monuron); 94(8):48349p. PA1 3-(substituted phenyl)-1,1-dimethylurea; 102(4): -25735f and 102(4):26023j. PA1 X is ##STR1## m has a value of at least 1 and is preferably 1-3; R and R" are independently H, methyl, or ethyl; PA1 n is 2-5; PA1 Y has valence m and is an organic radical.
In other attempts to hasten the cure of epoxy-dicyan-diamide systems, certain imidazole-isocyanate reaction products have been tried. European patent application 024,119 of July 21, 1980 describes a succinic acid salt of imidazole modified by phenylisocyanate which when combined with an epoxy resin of polyglycidyl ether, has a shelf life of only 2-3 days. U.S. Pat. No. 4,335,228 describes certain isocyanate blocked imidazoles with excellent shelf life, but which are useful only with epoxy resins that are solid at room temperature, a requirement that rules out use with some of the most useful epoxies. The accelerator claimed in U.S. Pat. No. 4,533,715 is naphthyldiisocyanate blocked with imidazole or certain imidazole derivatives. Shelf life of 6-8 months is claimed, along with cure times of 5 minutes at 250.degree. F. (with Epon 828). In U.S. Pat. No. Pat. No. 4,041,019 the epoxy curing agent is an imidazole capped with an isocyanate.
The instant invention concerns a novel class of imidazole compounds that serve as curing agents as well as curing accelerators for single-package epoxy thermosetting systems. The invention deals with the preparation and the application of these novel imidazoles for curing epoxy resins at an extremely rapid speed and at a reduced cure temperature without affecting the storage stability of the mixture of epoxy resin plus imidazole compound. Details are given below.